1. Field of the Invention
The present invention relates to a servo control system, and more particularly, is directed to a servo control system suitable for use in recording and/or reproducing heads of optical disk or magnetic disk units.
2. Related Background Art
In general, on an optical disk, magnetic disk or other rotating recording media, there are formed spiral or concentric tracks on which information is recorded. When information is recorded on the tracks or information stored on the tracks is reproduced, a tracking servo control and/or a focusing servo control may be carried out in order that a data recording head or reproducing head can properly trace the tracks.
FIG. 1 shows an example of the control system for accomplishing the tracking servo control, in which the reference numeral 1 denotes a tracking error detector (including an A/D converter) for detecting deviation data between an optical head and a target track (strictly speaking, deviation data between the light beam spot irradiated by the optical head and the target track), in other words, tracking error signals which indicate the error deviation between the optical head and the target track by means of, for instance, a well-known push-pull method. Also, the reference numeral 2 designates a digital compensator which operates or computes by use of tracking error signals the controlled variable of an actuator 4 for driving the optical head (the actuator which is referred to hereat indicates a tracking actuator and/or a focusing actuator). This digital compensator 2 operates at the predetermined sampling period the controlled variable of the actuator 4 in conformity with the given operation expressions so as to move the optical head to the target track based on the tracking error signals and outputs the results as the control signals into a drive circuit 3 (including a D/A converter). Thereby the drive circuit 3 drives the actuator 4 in accordance with the control signals to move the optical head to the target track. For each sampling, the control signal is thus output into the actuator 4 to move the optical head to the original target track immediately in case the optical head deviates from its target track, thus keeping the optical head in place on the track. That is to say, a servo control loop is constituted by the tracking error detector 1, the digital compensator 2, the drive circuit 3, and the actuator 4.
The same may be said of the focusing servo control.
In the conventional servo control system, however, the control variable is operated by the digital compensator without changing various parameters of the operation expression. As a matter of fact, however, due to changes in the environmental conditions such as secular change or temperature change of the actuator, the tracking capacity of the optical head for the target track is impaired. In this case, the foregoing parameters may be changed, whereas the parameters are initially set, so that it can not cope with such change in the environment. It was thus impossible to perform an exact tracking servo control and/or focusing servo control.